Witnesses describe the impact of the explosion in Beirut on Friday which killed Mohamad Chatah, a key adviser to the now exiled former Lebanese prime minister, Saad Hariri. The 62-year old is the second senior opposition figure to have been killed in the past 14 months. Chatah was an outspoken critic of the Syrian regime and of Hezbollah, which has held sway over the Lebanese government since Hariri was ousted three years ago. Fifteen other people were killed in the blast